Medical devices and methods of making the same

ABSTRACT

An endoprosthesis, such as a stent, having a layer that can enhance the biocompatibility of the endoprosthesis, and methods of making the endoprosthesis are disclosed.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit under 35 U.S.C. § 120 of U.S.application Ser. No. 10/629,934, filed on Jul. 29, 2003, the entirecontents of which are hereby incorporated by reference, which is acontinuation of U.S. application Ser. No. 10/263,212, filed on Oct. 2,2002, now U.S. Pat. No. 6,638,301, and this application is a divisionalapplication under 35 U.S.C. § 120 of U.S. application Ser. No.10/263,212, filed on Oct. 2, 2002, now U.S. Pat. No. 6,638,301, theentire contents of which are hereby incorporated by reference.

TECHNICAL FIELD

The invention relates to medical devices, such as, for example, stentsand stent-grafts, and methods of making the devices.

BACKGROUND

The body includes various passageways such as arteries, other bloodvessels, and other body lumens. These passageways sometimes becomeoccluded or weakened. For example, the passageways can be occluded by atumor, restricted by plaque, or weakened by an aneurysm. When thisoccurs, the passageway can be reopened or reinforced, or even replaced,with a medical endoprosthesis. An endoprosthesis is typically a tubularmember that is placed in a lumen in the body. Examples of endoprosthesisinclude stents and covered stents, sometimes called “stent-grafis”.

Endoprostheses can be delivered inside the body by a catheter thatsupports the endoprosthesis in a compacted or reduced-size form as theendoprosthesis is transported to a desired site. Upon reaching the site,the endoprosthesis is expanded, for example, so that it can contact thewalls of the lumen.

The expansion mechanism may include forcing the endoprosthesis to expandradially. For example, the expansion mechanism can include the cathetercarrying a balloon, which carries a balloon-expandable endoprosthesis.The balloon can be inflated to deform and to fix the expandedendoprosthesis at a predetermined position in contact with the lumenwall.

The balloon can then be deflated, and the catheter withdrawn.

In another delivery technique, the endoprosthesis is formed of anelastic material that can be reversibly compacted and expanded, e.g.,elastically or through a material phase transition. During introductioninto the body, the endoprosthesis is restrained in a compactedcondition. Upon reaching the desired implantation site, the restraint isremoved, for example, by retracting a restraining device such as anouter sheath, enabling the endoprosthesis to self-expand by its owninternal elastic restoring force.

To support a passageway open, endoprostheses are sometimes made ofrelatively strong materials, such as stainless steel or Nitinol (anickel-titanium alloy), formed into struts or wires. These materials,however, can be relatively radiolucent. That is, the materials may notbe easily visible under X-ray fluoroscopy, which is a technique used tolocate and to monitor the endoprostheses during and after delivery. Toenhance their visibility (e.g., by increasing their radiopacity), theendoprostheses can be coated with a relatively radiopaque material, suchas gold. Because the endoprostheses are typically kept in the body for arelatively long time, it is desirable that they have goodbiocompatibility.

SUMMARY

The invention relates to methods of making medical devices, such as, forexample, stents and stent-grafts, and methods of making the devices.More particularly, the invention features an endoprosthesis, such as astent, having a layer that can enhance the biocompatibility of theendoprosthesis.

In one aspect, the invention features a stent including a member havinga first portion, and a second portion disposed outwardly of the firstportion. The second portion is more radiopaque than the first portionand has a first layer including a radiopaque material, and a secondlayer defining an outer surface of the member and including theradiopaque material and a second material.

Embodiments may include one or more of the following features. Thesecond layer includes an alloy of the radiopaque material and the secondmaterial. The radiopaque material is selected from the group consistingof gold, platinum, palladium, and tantalum. The second material isselected from the group consisting of titanium, chromium, palladium,niobium, and silicon. The first portion includes a material selectedfrom the group consisting of stainless steel and nickel-titanium alloy.

The first portion can be the innermost portion of the member, and/orcontact the second portion.

The stent can further include a third portion between the first portionand the second portion, a polymeric layer on the member, and/or adrug-releasing layer on the member.

In another aspect, the invention features a stent including a memberhaving a first portion having a first layer including a radiopaquematerial, and a second layer defining an outer surface of the member andincluding the radiopaque material and a second material.

In another aspect, the invention features a stent including a memberhaving a first portion, and a second portion disposed outwardly of thefirst portion. The second portion is more radiopaque than the firstlayer and includes a first layer having a radiopaque material, and asecond layer including the radiopaque material and defining an outersurface of the member, the second layer having a lower oxidationpotential than an oxidation potential of the first layer.

Embodiments may include one or more of the following features. Theradiopaque material is selected from the group consisting of gold,platinum, palladium, and tantalum. The second layer includes an alloy ofthe radiopaque material and a second material. The second material isselected from the group consisting of titanium, niobium, palladium,chromium, and silicon.

The first portion can include a material selected from the groupconsisting of stainless steel and a nickel-titanium alloy. The firstportion can be the innermost portion of the member. The first portioncan contact the second portion.

The first and second portions can have different compositions.

The stent can further include a polymeric layer on the member and/or adrug-releasing layer on the member.

In another aspect, the invention features a stent having a member havinga first portion including a first layer comprising a radiopaquematerial, and a second layer comprising the radiopaque material anddefining an outer surface of the member. The second layer has a loweroxidation potential than an oxidation potential of the first layer.

In another aspect, the invention features a stent having a memberincluding a first portion having a concentration gradient of aradiopaque material, the first portion defining an outer surface of themember.

Embodiments may include one or more of the following features. Theconcentration of the radiopaque material increases as a function ofdistance from the outer surface. The concentration gradient variessubstantially linearly along a thickness of the first portion. Theradiopaque material is selected from a group consisting of gold,platinum, palladium, and tantalum. The first portion is formed of analloy including the radiopaque material and a second material. Themember further includes a second portion disposed inwardly of the firstportion, the second portion being more radiolucent than the firstportion.

In another aspect, the invention features a method of making a stentincluding a member. The method includes forming an outer layer on themember having a radiopaque material and a second material, and oxidizinga portion of the outer layer.

Embodiments may include one or more of the following features. Oxidizingthe portion includes forming an oxide or a nitride from the outer layer.The method further includes forming a radiopaque layer having theradiopaque material. The outer layer is formed with a compositionalgradient.

The outer layer is formed by a process selected from the groupconsisting of physical vapor deposition, chemical vapor deposition, andelectrodeposition.

Oxidizing the portion of the outer layer can be performed byelectropolishing, by heating the outer layer in an oxidizingenvironment, and/or by ion implanting oxygen in the outer layer andheating the outer layer.

The method can further include forming a polymeric layer on the outerlayer, and/or forming a drug-releasing layer on the outer layer.

Other aspects, features and advantages of the invention will be apparentfrom the description of the preferred embodiments thereof and from theclaims.

DESCRIPTION OF DRAWINGS

FIG. 1 is a perspective view of an embodiment of a stent.

FIG. 2 is a schematic, cross-sectional view of the stent of FIG. 1,taken along line 2-2.

FIG. 3 is a schematic, cross-sectional view of a strut of an embodimentof a stent,

FIG. 4 is a schematic, partial cross-sectional view of a strut of anembodiment of a stent.

FIG. 5 is a schematic diagram of an embodiment of an ion beam assisteddeposition system.

FIG. 6 is a plot of material concentration as a function of time.

FIG. 7 is a table of parameters for an ion beam assisted depositionprocess.

FIG. 8 is a table of parameters for an ion beam assisted depositionprocess.

FIG. 9 is a table of parameters for an ion beam assisted depositionprocess.

DETAILED DESCRIPTION

FIG. 1 shows a support 12 carrying a stent 10, which is in the form of atubular member defined by struts 11 and openings 13. Depending on thetype of stent 12 (e.g., balloon-expandable or self-expandable), support12 can be a balloon catheter or a catheter shaft. Referring to FIG. 2,stent 10 includes multiple cross-sectional portions. In particular,struts 11 of stent 10 are formed of a relatively radiolucent core 14surrounded by a relatively radiopaque portion 16. Radiopaque portion 16includes a radiopaque layer 18, e.g., made of gold, and a layer 20,e.g., made of a gold-titanium alloy, that can enhance thebiocompatibility of stent 10. For example, layer 20 can be passivated toprovide stent 10 with a relatively inert outer surface.

In general, stent 10 can be formed by coating a relatively radiolucentstent with a radiopaque material, such as gold or platinum, to formlayer 18. Layer 20 is then formed on the radiopaque material. Layer 20can be formed on the pre-formed radiopaque layer 18 and/or formed from aportion of the radiopaque layer. Layer 20 is then passivated, e.g., byforming a layer of an oxide or nitride on layer 20 or by convertinglayer 20 to an oxide or a nitride.

Core 14 is generally formed of one or more core material selected toprovide stent 10 with certain physical and mechanical properties. Forexample, the core material is selected to provide stent 10 withsufficient hoop strength and radial strength so the stent can maintain abody vessel open. Suitable core materials include stainless steel (e.g.,316L stainless steel), Nitinol (e.g., for self-expandable stents), othertitanium alloys, tantalum alloys, zirconium alloys, and/or niobiumalloys. At the same time, it is also desirable to reduce (e.g.,minimize) differences or mismatch in mechanical properties (e.g.,stiffness) between the stent and the body vessel. The mechanicalmismatch can cause, for example, inflammation and/or re-occlusion of thevessel. One method of reducing mechanical mismatch is to form the stentwith less material (e.g., by forming smaller struts 11), therebyapproximating the compliancy or resiliency of the vessel. However,reducing the amount of core material in stent 10 can also reduce theradiopacity of the stent.

To increase the radiopacity of stent 10, the stent includes radiopaqueportion 16 disposed over core portion 14. Portion 16 includes radiopaquelayer 18, which is formed with a radiopaque material. The radiopaquematerial can be any material with a density and/or linear absorptioncoefficient sufficient to enhance the radiopacity of stent 10. Inembodiments, the radiopaque material has a density and/or linearabsorption coefficient to attenuate an incident X-ray beam. In somecases, the radiopaque material has a density of equal to or greater thanabout 10 g/cc. Examples of radiopaque materials include gold, platinum,palladium, tantalum, iridium, cobalt, titanium, tungsten, stainlesssteel, Nitinol, and metal alloys containing a sufficient percentage ofheavy elements. Radiopaque layer 18 can be, for example, up to about 8microns thick, e.g., about 6-8 microns, thick. Methods of formingradiopaque layer 18 include, for example, electrodeposition, physicalvapor deposition (e.g., sputtering), chemical vapor deposition,galvanizing, and/or dipping (e.g., in molten material).

In some cases, however, the radiopaque materials do not have a desiredlevel of biocompatibility and/or the biocompatibility of the material isunknown (e.g., in the long term). It is believed, for example, that goldmay affect (e.g., catalyze) electron transfer in certain undesirablereactions in the body. Accordingly, radiopaque portion 16 includes arelatively inert layer 20 disposed over radiopaque layer 18.

Layer 20 enhances the biocompatibility of stent 10 by providing thestent with a layer (as shown, an outer layer) that can be passivated,e.g., more easily than radiopaque layer 18.

For example, layer 20 is capable of reacting (e.g., oxidizing) andforming products, such as oxides, nitrides, and/or carbides, that aremore inert, and therefore, more biocompatible, than the material(s) inradiopaque layer 18. Relative to radiopaque layer 18, layer 20 has alower oxidation potential, i.e., can be more easily oxidized to form abiocompatible product.

In some embodiments, layer 20 includes a mixture (here, an alloy) of theradiopaque material(s) in radiopaque layer 18 and one or more alloyingmaterial. The alloying material can be any material capable of forming amixture with the radiopaque material(s), and forming a product that ismore easily passivated than the radiopaque material(s). The alloyingmaterial can be, for example, tantalum, titanium, niobium, zirconium,chromium, silicon, rhodium, iridium, platinum, and/or palladium. Any ofthe alloying materials can be used with any of the radiopaque materialsdescribed above.

As an example, for a gold radiopaque layer 18, the alloying material canbe titanium. In this example, layer 20 includes an alloy ofgold-titanium, such as Au_(0.30)Ti_(0.70), which can be more easilypassivated than gold. That is, relative to gold, the gold-titanium alloycan more easily form or be converted to a product, e.g., an oxide, thatis relatively inert and biocompatible. In embodiments, for the alloy ofgold-titanium (Au_(x)Ti_(y)) x can range from about 0-30%, and y canrange from about 70-100%. For example, x can be equal to or greater thanabout 0%, 5%, 10%, 15%, 20%, or 25%, and/or equal to or less than about30%, 25%, 20%, 15%, 10%, or 5%. In embodiments, the concentration oftitanium, y, can be equal to or greater than about 70%, 75%, 80%, 85%,90%, or 95%, and/or less than or equal to 100%, 95%, 90%, 85%, 80%, or75%. Layer 20 can be up to about 10 microns thick, e.g., about 0.1-10microns thick. Temary (e.g., Au—Ti—Cr) or higher mixtures or alloysystems can be formed.

In some embodiments, layer 20 can be formed on a pre-formed radiopaquelayer 18.

For example, after radiopaque layer 18 is formed, modified layer 20 canbe applied on the radiopaque layer by physical vapor deposition,including sputtering and ion beam assisted deposition, chemical vapordeposition, or electrodeposition. Layer 20 can also be formed by forminglayers, e.g., alternating layers, of the radiopaque material and thealloying material on layer 18 in a predetermined ratio, and heating thelayers (e.g., at elevated, annealing temperatures) to form the alloy bydiffusion.

Alternatively or in addition, layer 20 can be formed from a portion of aformed radiopaque layer 18. That is, a portion of the radiopaque layer18 can be converted to layer 20. For example, a gold-titanium layer 20can be formed by implanting titanium ions into a formed gold radiopaquelayer 18, and annealing the radiopaque layer. As a result, a certainthickness of the radiopaque layer (e.g., in the sub-micron range) isconverted to an alloyed modified layer that can be passivated. Inanother example, a layer of alloy material, e.g., Ti, can be depositedon radiopaque layer 18, e.g., Au, and the layers can be heated, e.g.,annealed, to form an alloy, e.g., Au—Ti.

It should be noted that while FIG. 2 shows radiopaque layer 18 and layer20 as two discrete, well-defined layers, in some embodiments, theinterface between the layers is not well defined. As a result, theendoprosthesis can be formed with good adhesion and high durability(e.g., reduced risk of flaking). Corrosion from contact of dissimilarmaterial can also be reduced. The interface may not be well defined, forexample, when modified layer 20 is formed from a formed radiopaque layer18.

In some embodiments, radiopaque portion 16 does not include an interfacebetween two layers. Referring to FIG. 3, a strut 22 of a stent is formedof a relatively radiolucent core 24 surrounded by a relativelyradiopaque layer 26. Core 24 is generally the same as core 14 describedabove. Radiopaque layer 26 includes one or more radiopaque material andone or more alloying material, as described above. In addition,radiopaque layer 26 is formed having a compositional gradient in whichthe concentration(s) of the alloying material(s) and/or the radiopaquematerial(s) varies along the thickness of layer 26 (arrows A and B). Asan example, for a radiopaque layer 26 formed of a gold-titanium alloy,layer 26 can be relatively gold-rich (or titanium-poor) at surface 28adjacent to core 24, and relatively gold-poor (or titanium-rich) atouter surface 30. At surface 28, the concentration of the radiopaquematerial can be about 100%; and at outer surface 30, the concentrationof the alloying material can be about 100%. The concentration(s) of theradiopaque material(s)l and/or the alloying material(s) can varylinearly or non-linearly (e.g., exponentially) between surfaces 28 and30. The concentration(s), e.g., of the alloying material, can increaseor decrease from surface 28 to surface 30. In certain embodiments, layer26 having the compositional gradient can be formed on a radiopaquelayer, such as radiopaque layer 18.

Methods of forming compositionally-graded layer 26 include usingphysical vapor deposition while controlling the source of materials usedfor deposition. In another method, layer 26 can be formed by formingalternating layers of a radiopaque material and an alloying material ina predetermined ratio, and annealing the layers. For example, referringto FIG. 4, to form a concentration gradient of titanium along layer 26,layers of titanium 27 a, 27 b, and 27 c can be formed alternating withlayers of gold 29 a, 29 b, and 29 c. Titanium layer 27 a is thicker thanlayer 27 b, which is thicker than layer 27 c. Gold layers 29 a-29 c areof equal thickness. When the layers are subsequently annealed, they candiffuse together and form a gold-titanium alloy in which theconcentration of titanium varies along the thickness of layer 26 (here,increasing with increasing distance from core 24).

After layer 20 or 26 is formed, stent 10 can be passivated by exposingthe stent to an appropriate environment. For example, stent 10 can beoxidized by heating the stent in an oxidizing atmosphere, such as onecontaining oxygen and/or water, to form an oxide layer on layer 20 or26. Nitrides can be formed by heating stent 10 in an atmospherecontaining nitrogen, nitrogen-hydrogen, and/or ammonia. Carburizing,e.g., increasing the surface concentration of carbon, can be performedby exposing stent 10, at an elevated temperature, to an atmosphere richin a hydrocarbon gas, such as methane. Alternatively or in addition,passivation can be performed by electropolishing to produce anoxide-rich surface layer. In some cases, passivation can occurrelatively spontaneously, e.g., upon exposure to air, when the oxidationpotential is relatively low.

Stent 10 can then be finished, e.g., electropolished to a smooth finish,according to conventional methods. Stent 10 can be finished beforepassivation. Alternatively, stent 10 can be formed textured.

Stent 10 can then be used, e.g., delivered and expanded, according toconventional methods.

Generally, stent 10 can be self-expandable, balloon-expandable, or acombination of both. Examples of stent 10 and support 12 are describedin U.S. Pat. No. 5,725,570 (Heath) and U.S. Pat. No. 5,234,457(Andersen), all hereby incorporated by reference.

In other embodiments, stent 10 is a part of a stent-graft. Thestent-graft can be a stent attached to a biocompatible, non-porous orsemi-porous polymer matrix made of polytetrafluoroethylene (PTFE),expanded PTFE, polyethylene, urethane, or polypropylene. Stent 10 caninclude a releasable therapeutic agent or a pharmaceutically activecompound, such as described in U.S. Pat. No. 5,674,242, andcommonly-assigned U.S. Ser. No. 09/895,415, filed Jul. 2, 2001, allhereby incorporated by reference. The therapeutic agents orpharmaceutically active compounds can include, for example,anti-thrombogenic agents, antioxidants, anti-inflammatory agents,anesthetic agents, anti-coagulants, and antibiotics.

The following examples are illustrative and not intended to be limiting.

EXAMPLE

The following example describes ion beam assisted deposition (IBAD) as amethod for depositing thin films on a substrate, e.g., a stent.

Referring to FIG. 5, an IBAD system 50 generally includes a fixtureassembly 52 configured to support a stent 54, and a deposition assembly56. System 50 is used in a vacuum chamber 51 at pressures of about1×10⁻⁴−3×10⁻⁴ Torr, provided in part by a diffusion pump 58.

Deposition assembly 56 includes two crucibles 60 and 62, theirrespective shutters 64 and 66, two electron beam evaporators 68 and 70,and an ion beam gun 72. Crucibles 60 and 62, e.g., made of graphite,contain materials to be deposited, such as gold and titanium. Electronbeam evaporators 68 and 70 are configured to generate a flow ofelectrons that can be focused (e.g., using magnetic fields) on thematerials in crucibles 60 and 62, respectively, to melt and to evaporatethe materials to form thermally evaporated materials 76. Evaporators 68and 70 can have water-cooled jackets that cool crucibles 60 and 62,respectively. Ion beam gun 72 is configured to receive a flow of argon(e.g., 2-4 sccm) and to ionize the argon to form a plasma 74. Plasma 74is accelerated out of ion beam gun 72 to stent 54 using magnets (notshown). Shutters 64 and 66 can be moved, e.g., swiveled, to allow or toblock the flow of evaporated material 76 from crucibles 60 and 62,respectively.

Fixture assembly 52 is generally configured to allow stent 54 to beuniformly coated with evaporated material 76. Typically, the thermalevaporation process can deposit a film of material 76 on a substratethat is in a line of sight of crucible 60 or 62. To provide uniformcoverage on stent 54, the stent is rotated during deposition. Inembodiments, stent 54 is placed on a rotatable spindle. The frictionbetween the stent and the spindle can hold the stent in place duringrotation to provide a coated stent without contact points.Alternatively, stent 54 can be clipped to a rotatable shaft.

A quartz crystal 78 is used to determine the thickness of the depositedmaterial. Crystal 78 is interfaced to a controller (not shown) andoscillated. The controller is calibrated such that the thickness ofmaterial deposited on crystal 78 (and thus also stent 54) can becalculated by measuring the change in the oscillation frequency of thecrystal.

A method of coating using IBAD will now be described.

Stent 54, e.g., a Nitinol or stainless steel stent, is thoroughlychemically cleaned. For example, stent 54 can be cleaned in a solvent(such as isopropyl alcohol or acetone) and a degreaser, and rinsed withdeionized water. Heat and/or agitation, e.g., using ultrasonic energy,can be used to clean stent 54. Stent 54 is then placed on fixtureassembly 52, which is then placed in vacuum chamber 51, with the stentabout two feet from crucibles 60 and 62.

Stent 54 is then subjected to a sputter cleaning. Chamber 51 isevacuated to a pressure of about 1×10⁻⁵ Torr, and ion beam gun 72 isactivated. Ion beam gun 72 ionizes argon gas to form plasma 74, and theplasma is accelerated to stent 54 to sputter clean/etch the surface ofthe stent. The angle of incidence for plasma 74 can be about 45-90°,e.g., about 70°. In embodiments, stent 54 is sputter cleaned for about20-30 minutes. An estimated 100-300 angstroms of material can beremoved.

A first material, e.g., gold in crucible 60, is then deposited. Duringthe final ten minutes of sputter cleaning, electron beam evaporators 68and 70 are slowly ramped up. Shutters 64 and 66 are over theirrespective crucibles 60 and 62, so no material can deposit on stent 54.After sputter cleaning is complete and the material to be deposited ismolten, shutter 64 moves, e.g., swivels, to allow evaporated material tocoat stent 54. The surface of stent 54 is simultaneously bombarded withplasma 74. It is believed that as ions of the first material deposit onstent 54, plasma 74 transfers energy to the ions, freeing some ions fromthe surface of the stent and allowing some ions to migrate on the stentsurface. As a result, it is believed that a composite including thefirst material is formed with enhanced density.

A second material, e.g., titanium, tantalum, or platinum, is thendeposited. After the thickness of the first material coated on stent 54reaches, e.g., about 200-500 angstroms, shutter 66 is moved to allow thesecond material (in crucible 62) to co-deposit with the first material.The concentrations of each material can be controlled by adjusting thepower to evaporators 68 and 70. For example, referring to FIG. 6,initially the concentration of the first material is relatively high,and the second material is then slowly introduced. In embodiments, attime t, shutter 64 is moved to prevent the first material fromdepositing on stent 54, and a pure layer of the second material isdeposited over the alloy layer (i.e., the layer having the first andsecond materials). Then, stent 54 is allowed to cool, chamber 51 isreturned to atmospheric pressure, and the stent is removed from thechamber.

In embodiments, stent 54 is then annealed. Annealing can promotediffusion between the layers of materials and/or the layers and thestent substrate, and can strengthen bonding or adhesion between thelayers. In some cases, a Nitinol stent can be annealed at about 300-400°C., and a stainless steel stent can be annealed at about 500-1000° C.Annealing times can vary, e.g., from a few minutes to days, depending,for example, on the diffusion of the materials in stent 54, which can betemperature-dependent.

FIG. 7 shows ranges for some process parameters.

A stent was coated with titanium using the procedures described above.The process parameters are shown in FIG. 8.

A stent was coated with a platinum-gold using the procedures describedabove. The process parameters are shown in FIG. 9. The platinum-goldgradient was similar to that shown in FIG. 6.

OTHER EMBODIMENTS

In other embodiments, one or more intermediate layers can be formedbetween core 14 or 24 and radiopaque layer 18 or 26, i.e., at least aportion of the core and the radiopaque layer do not contact. Forexample, in embodiments in which there is lattice mismatch between thecore and the radiopaque layer, intermediate layer(s) can be selected tohave intermediate lattice parameters to serve as buffer layer(s),thereby reducing (e.g., minimizing) stress between the core and theradiopaque layer. The intermediate layer(s) can be, for example, amixture of the core material and the radiopaque material.

Layer 20 may not include the radiopaque material(s) in radiopaque layer18. For example, a radiopaque layer may include gold, while layer 20includes a material that can be passivated, such as a platinum-titaniumalloy.

Radiopaque layer 18, layer 20, and/or layer 26 can cover all or only oneor more selected portions of a stent. For example, radiopaque layer 18,layer 20, and/or layer 26 may be formed only on one or more end portionsof the stent.

In some embodiments, other types of layers can be formed on layer 20 or26. For example, one or more selected portions of a stent may include amagnetopaque (i.e., visible by magnetic resonance imaging (MRI))material on layer 20 or 26. Suitable magnetopaque materials include, forexample, non-ferrous metal-alloys containing paramagnetic elements(e.g., dysprosium or gadolinium) such as terbium-dysprosium, dysprosium,and gadolinium; non-ferrous metallic bands coated with an oxide or acarbide layer of dysprosium or gadolinium (e.g., Dy₂O₃ or Gd₂O₃);non-ferrous metals (e.g., copper, silver, platinum, or gold) coated witha layer of superparamagnetic material, such as nanocrystalline Fe₃O₄,CoFe₂O₄, MnFe₂O₄, or MgFe₂O₄; and nanocrystalline particles of thetransition metal oxides (e.g., oxides of Fe, Co, Ni).

In other embodiments, radiopaque layer 18, layer 20, and/or layer 26 maybe formed on medical devices other than stents and stent-grafts, forexample, those where radiopacity is desired such as orthopedic implants.

All publications, applications, and patents referred to herein areincorporated by reference in their entirety.

Other embodiments are within the claims.

1. A method of making a stent including a member, the method comprising:forming an outer layer of the stent on the member, the outer layer ofthe stent comprising an alloy comprising a radiopaque metal element anda second metal element, wherein the radiopaque metal element is selectedfrom a group consisting of gold, palladium, and platinum; and oxidizinga portion of the alloy of the outer layer.
 2. The method of claim 1,wherein oxidizing the portion comprises forming an oxide from the outerlayer.
 3. The method of claim 1, wherein oxidizing the portion comprisesforming a nitride from the outer layer.
 4. The method of claim 1,further comprising forming a radiopaque layer comprising the radiopaquemetal element.
 5. The method of claim 1, wherein the outer layer isformed with a compositional gradient.
 6. The method of claim 1, whereinthe outer layer is formed by a process selected from the groupconsisting of physical vapor deposition, chemical vapor deposition, andelectrodeposition.
 7. The method of claim 1, wherein oxidizing theportion of the outer layer is performed by electropolishing.
 8. Themethod of claim 1, wherein oxidizing the portion of the outer layer isperformed by heating the outer layer in an oxidizing environment.
 9. Themethod of claim 1, wherein oxidizing the portion of the outer layer isperformed by ion implanting oxygen in the outer layer and heating theouter layer.
 10. The method of claim 1, further comprising forming apolymeric layer on the outer layer.
 11. The method of claim 1, furthercomprising forming a drug-releasing layer on the outer layer.
 12. Themethod of claim 1, wherein the radiopaque metal is capable ofattenuating an incident X-ray beam by more than about 70%.
 13. Themethod of claim 1, wherein the second metal element is selected from thegroup consisting of titanium, chromium, palladium, niobium, and silicon.14. The method of claim 1, wherein the member comprises a materialselected from the group consisting of stainless steel andnickel-titanium alloy.
 15. The method of claim 1, wherein the outerlayer defines an outer surface of the member.
 16. The method of claim 1,further comprising forming a second layer comprising the radiopaquemetal element, the second layer disposed inwardly of the outer layer.17. The method of claim 16, wherein the outer layer has a loweroxidation potential than an oxidation potential of the second layer. 18.A method of making a stent including a member, the method comprising:forming a first layer of a stent comprising a radiopaque metal elementon the member, wherein the radiopaque metal element is selected from thegroup consisting of gold, platinum, and palladium; forming a secondlayer of a stent comprising an alloy of the radiopaque metal element anda second metal element; oxidizing a portion of the alloy of the secondlayer.